1. Field of the Invention
This invention relates generally to portable testers for measuring the water content of a material using a measurement of the dielectric constant of the material and particularly to such testers, which measure the moisture content of a predetermined repeatable volume of grain.
2. Description of the Prior Art
There have previously been several ways to compensate for how the grain sits in the test cell or a means to account for the grain density to increase the moisture measurement accuracy. The early prior art grain moisture meters, both portable and commercial, neither used preset compaction nor weighing of the grain to allow for density variation compensation. The lack of these compensations limited the accuracy in their moisture measurements.
Later commercial testers weighed the grain in the test cell for density compensation. However, this form of density compensation is not easily incorporated into small portable moisture measuring devices and is impractical for such devices. Weighing grain, nevertheless, today is a known method for large on site commercial moisture testers to compensate for grain density variations.
Compaction is used to determine grain moisture as shown in U.S. Pat. No. 3,890,830, which utilized a compaction method for determining the grain moisture. However, this technique provides no measurement of the dielectric constant of the grain after compaction. Not obtaining the dielectric constant measurement in conjunction with the compaction measurement results in an unacceptable error when determining the grain moisture content. It also fails to provide a preset compaction trigger point to determine when the grain in the test cell is properly compacted and ready for the dielectric constant measurement.
Another known portable moisture tester using density, or compaction control is found in U.S. Pat. No. 5,663,650. It used a spring-loaded cap with a plunger inside of it. The procedure involved placing grain in the test cell and screwing down the cap until a detent, in the middle of the cap, became flush with the top of the cap. After the detent was flush, the user would then manually press the TEST button to obtain the moisture content. While a useful and valid approach for grain compaction repeatability, this device left it to the users discretion to determine when the cap detent was flush with the top of the cap before testing dielectric constant of the grain in the test cell. The users estimation of the flushness of the detent with the top of the cap is a subjective judgment and introduced errors. This human intervention did not allow for an automatic moisture reading trigger point since the user makes the detent flush then manually presses the test button to measure the grain""s dielectric constant. Also, this method made it the responsibility of each user to determine when the detent is indeed flush with the cap. This resulted in measurement variations and error from one user to the next.
In view of the forgoing it will be seen that the prior art portable grain moisture measurement devices did not provide a density dependent automatic moisture measurement trigger point which was repeatable and error free from user to user.
The present invention provides a portable grain moisture-measuring device having a means of measuring the compressive force of the grain in a test cell of the device via a strain gage-bending arm on the bottom of the test cell. The signal from the strain gage is compared to a preset signal of desired force to automatically take the grain moisture measurement at the same amount of grain compaction for each successive test. The strain gage is in a full Wheatstone bridge configuration with the differential output of the bridge amplified by a gain of 1000 to obtain a reasonable resolution for a device microprocessor used to operate the automatic testing triggering and other measurements.
When using the device, the test cell is filled directly by the user. After that the user simply threads the cap and stops turning the cap when the tester beeps and displays TESTING. This indicates the preset force on the grain has been achieved and testing measurements have been initiated. If the user continues to screw down the cap, after the beep, this does not produce any error since the dielectric constant reading immediately follows the strain gage trigger point, within milliseconds.
The present device uses a firmware algorithm that first determines the unloaded strain level and then continuously measures the strain, as the cap is threaded. Once the measurement reaches the preset difference from the initial strain the tester knows it is at the same compaction level as when the grain was originally calibrated in the test cell. This results in a more repeatable measurement.
The device of the present invention uses measurements of compaction, temperature, and dielectric constant to obtain the grain moisture. More specifically, it uses a preset compaction trigger point to determine when the grain in the test cell is ready for the above measurements including the dielectric constant measurement. The automatic trigger point is preset and is proportional to the amount of compression on the grain within the filled test cell. This preset automatic triggering of testing allows the moisture meter to identify the correct time to make repeatable error free measurements including the dielectric constant of the grain filled test cell. This trigger point is preset to be the same point used when calibrating the testers memory for different grains to provide programmed grain moisture calibration curves. This results in repeatable moisture readings each time the grain moisture meter is used even by different people.
In view of the foregoing it will be seen that one aspect of the present invention is to provide a grain moisture-measuring device, which automatically initiates moisture-testing measurements at preset grain compaction levels.
Another aspect is to provide a repeatable moisture measurement between different tester users.
Yet another aspect is to provide a tester having a triggered moisture measurement at a preset compaction of a test cell, which is not affected, by further compaction of the cell.